This invention is directed to various improvements by which high grade energy is recovered in the contact process for the manufacture of sulfuric acid. More particularly, the invention is directed to improvements in which the energy of low pressure steam is recovered at high temperature, the vapor phase heat of formation of sulfuric acid in wet conversion gas is recovered in an economizer, and absorption heat is used to preheat oxygen-containing gas used for combustion of a source of sulfur in generation of sulfur dioxide to be fed to the converter. The energy recovered in the latter step is upgraded by transfer of heat from the sulfur dioxide combustion gas to a high temperature heat transfer fluid in a waste heat recovery unit.
Until recently, only about 55% to 60% of the heat generated in the contact sulfuric acid process was recovered in useful form. A major improvement in energy recovery has been provided in the processes of McAlister and Ziebold U.S. Pat. Nos. 4,576,813 and 4,670,242, and McAlister copending application Ser. No. 369,301, filed June 21, 1989, all of which describe processes for the recovery of the heat of absorption in the form of medium pressure steam. In the heat recovery system described in these disclosures, an absorption tower is operated at high temperature and heat is transferred from the absorption acid to produce medium pressure steam. By maintaining the acid concentration in the range typically of 99% to 100%, alloy heat exchangers may be used for recovery of the absorption heat. Practice of the McAlister and Ziebold processes allows process heat energy recovery capability to be increased to the range of 90 to 95%. The process of pending Ser. No. 369,301, which is expressly incorporated herein by reference, describes the particular application of absorption heat recovery to a process in which sulfuric acid is produced from a wet sulfur dioxide-containing gas.